1. Field of the Invention
The present invention relates to an amplification circuit, and in particular, to a power amplifier of a bridge tied load (BTL) system for audio (a BTL power amplifier), which is used for, for example, a power amplifier for car audio of a high output specification.
2. Description of the Related Art
In a conventional BTL power amplifier, an input signal Vin is subjected to voltage-current conversion (V-I conversion) at a gm amplifier serving as a preamplifier to be made into a BTL signal current, and the BTL signal current is output to a circuit formed of first and second resistance elements connected in series. The series-connection node of the first and second resistance elements are connected to a reference input terminal. A voltage generated across the first resistance element is amplified by a first operational amplifier, and is output from the first operational amplifier as a same phase output voltage VoutP having the same phase as the input signal Vin. On the other hand, a voltage generated across the second resistance element is amplified by a second operational amplifier, and the amplified voltage is output from the second operational amplifier as an opposite phase output voltage VoutM having an opposite phase to the input signal Vin. A closed loop gain GvP of the first operational amplifier and a closed loop gain GvM of the second operational amplifier can be given by the following expressions (1) and (2).
                    GvP        =                              VoutP            Vin                    =                                    gm              ·              R                        ⁢                                                  ⁢                          3              ·                                                                    R                    ⁢                                                                                  ⁢                    1                                    +                                      R                    ⁢                                                                                  ⁢                    2                                                                    R                  ⁢                                                                          ⁢                  2                                                                                        (        1        )                                GvM        =                              VoutM            Vin                    =                                                    -                gm                            ·              R                        ⁢                                                  ⁢                          6              ·                                                                    R                    ⁢                                                                                  ⁢                    4                                    +                                      R                    ⁢                                                                                  ⁢                    5                                                                    R                  ⁢                                                                          ⁢                  5                                                                                        (        2        )            
Assuming that feedback quantity of the first operational amplifier and the second operational amplifier are defined as f1 and f2, respectively, f1 and f2 can be given by the following expressions (3) and (4).
                              f          ⁢                                          ⁢          1                =                              R            ⁢                                                  ⁢            2                                              R              ⁢                                                          ⁢              1                        +                          R              ⁢                                                          ⁢              2                                                          (        3        )                                          f          ⁢                                          ⁢          2                =                              R            ⁢                                                  ⁢            5                                              R              ⁢                                                          ⁢              4                        +                          R              ⁢                                                          ⁢              5                                                          (        4        )            
The feedback quantity do not depend on a frequency and is at a fixed value, and thus the BTL power amplifier of the above configuration is excellent in oscillation stability. In addition, since the circuit configuration of the first operational amplifier is the same as that of the second operational amplifier, the BTL power amplifier according to the above configuration has advantages that the circuit structure can be made symmetric and the circuit designing can be made easy.
However, in the power amplifier configured as described above, a noise and an offset voltage generated in the gm amplifier are amplified by the first operational amplifier and the second operational amplifier. Thus, the power amplifier as described above is unsuitable for use in a power amplifier of a low noise and a low offset voltage. Particularly, in the case of applying the circuit of the above configuration to an audio power amplifier or the like which is made into an integrated circuit, a sound quality is deteriorated if the noise is large, and a POP noise becomes large if the offset voltage is large. In order to attain a low offset voltage, high accuracy is required for elements forming the gm amplifier, which increases the chip area of the gm amplifier.
A concrete example of the gm amplifier, which is a preamplifier in the BTL power amplifier circuit of the above configuration, is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 4-343506.